In motor applications it may be suitable to control the current energizing the motor winding(s). This current control may be used to limit the max current, produce a specific current less than the max value, or to produce a specific current profile over time. There are many different types of motors, brushed DC, brushless DC, induction, linear induction, linear synchronous, stepper, etc.
Stepper motors are very common in applications needing position control without requiring feedback sensors (open loop control). ATMs, surveillance cameras, printers, scanners, robotics and office automation are some such applications using stepper motors. A stepper motor has electromagnets (windings) that control its movement. To make the motor shaft turn, the electromagnets are energized in a controlled manner using a driver, or the like, which may be an Integrated Circuit (IC), or the like.
In a four-magnet stepper motor a first electromagnet is turned on, attracting the nearest teeth of a gear-shaped iron rotor. With the teeth aligned to this first electromagnet; they will be slightly offset from a second electromagnet. The first electromagnet is then turned off, and the second electromagnet is energized, pulling the teeth into alignment with it. This results in rotation, typically on the the order of a few degrees. A third electromagnet may then be energized and another rotation of the same magnitude occurs, and then the fourth electromagnet is energized for an addition rotation of the same amount. When the first electromagnet is again enabled, the rotor will have rotated by one tooth position. Hence, such a four magnet stepper motor may rotate a full rotation in a number of steps equal to four times the number of teeth on the rotor. To have smother motion, more teeth may be added, making each step have a smaller arc length. This approach to smoother higher quality motion is costly and has a physical limit with respect to teeth size and production practicalities.
However, in micro-stepping, instead of increasing the number of teeth, orthogonal electrical fields are ratioed (i.e. modified or multiplied by a ratio) to produce a vector sum of forces producing a stable location in-between full steps of the teeth. This is implemented using micro-stepping drivers. Therein time domain currents for orthogonal motor electromagnets windings (coil) are controlled to produce sinusoidal profiles that are ninety degrees out of phase. Each step is associated with a certain amount of current through each electromagnet coil and results in a particular position of the motor. With each step, the current profile advances to the next step making the motor move to the next step. At any point in time the magnitude of the currents will produce a vector sum on a one hundred percent constant torque circle, such that within every full step/tooth the motion may be broken down into equal portions of smaller steps. Within each discrete step of holding current the coil current must be regulated to the specific value.
Pulse Width Modulation (PWM) chopping may be used for current regulation. An H-bridge consisting of a number of Laterally Diffused Metal Oxide Semiconductor (LDMOS) transistors may be used for switching a power supply into and out of the circuit controlling the stepper motor by the nature of a low impedance path or high impedance path through a switch employed for driving a motor winding. Each coil is usually driven by an H-bridge circuit. During drive, a high side FET (i.e. LDMOS transistor) on one side of the coil and a low side FET on the other side of the coil are turned on. If a method for stopping the current from building in the coil is not employed, the driver IC and motor will likely be damaged. The method used is commonly referred to as decaying the current, or recirculation of the current. There are three implementations of decay, fast decay, slow decay and mixed decay. Fast decay is placing an opposing polarity voltage across the winding (i.e. reversing the voltage across the coil). This results in a current decay rate which is the same as the charge rate for the coil. Allowing the current to flow into two low-side transistors (i.e. the current is recirculated using two low side FETs) is referred to as slow decay, which results in a slower decay rate than fast decay. Both fast and slow decay may be used in the same cycle, which is then referred to as mixed decay. For example, when a desired regulation current is reached, first fast decay may be performed, followed by slow decay.
A motor system has many different variables contributing to making the correct decay scheme vary, system to system. When a system integrates a new motor, motor driver, or both into their application it must be tuned to apply the correct decay scheme, such as by observing the current profile on an oscilloscope. This may be a tedious, time consuming, difficult process, requiring particular skills and/or training, particularly where multiple-sourcing may be employed for motor system components. For example, the correct decay settings may depend own individual motor load, supply voltage, PWM frequency, current being sourced, speed of rotation, back electromotive force (BEMF), etc. Depending on such factors, some tradeoffs are also present when attempting to derive a decay scheme that will satisfy all currents (low currents and peak currents). For example, the decay mode that is best for reducing ripple is not the best decay scheme to regulate small currents. When one decay mode is selected, the setting may become suboptimal as the situation changes (e.g. battery supply lowers, motor characteristics change, step frequency changes, etc.).
Slow decay is typically not good for regulating low levels of current. Often, the slow decay rate cannot discharge the current built up during a minimum motor on time, resulting in current run-off. Fast decay is preferred for this case. However, while regulating larger current, fast decay results in larger ripple due to the charge/discharge rate being the same. For fast step response, fast decay is typically preferred. However, once a holding current is reached, undershoot and larger ripple may result. Slow decay is preferred for reducing ripple but results in longer step response time.
For battery-powered applications in particular, initial decay settings may become sub-optimal as supply voltage droops. As a motor ages and becomes more resistive, initial decay setting will need to be adjusted. Fixed decay schemes typically cannot handle BEMF well and may result in repeated patterns in current regulation that fall in the audible frequency range leading to noisy motor operation. Slow decay setting is more efficient but has drawbacks of longer step response, inability to hold low current, etc. Fast decay solves these problems but is less efficient due to switching losses and has more ripple. Insufficient decay may be provided at low currents, resulting in loss of current regulation and increased harmonic distortion, or correct decay at low currents may result in far too much decay at higher currents. Hence, typical fixed decay schemes characteristically arrive at a non-optimized solution for all factors involved in motor control.